The p53 gene is one of the most studied and well-known genes. p53 plays a key role in cellular stress response mechanisms by converting a variety of different stimuli, for example, DNA damage, deregulation of transcription or replication, and oncogene transformation, into cell growth arrest or apoptosis (T. M. Gottlieb et al., Biochem. Biophys. Acta, 1287, p. 77 (1996)).
p53 has a short half-life, and, accordingly, is continuously synthesized and degraded in the cell. However, when a cell is subjected to stress, p53 is stabilized. Examples of cell stress that induce p53 stabilization are:                a) DNA damage, such as damage caused by UV (ultraviolet) radiation, cell mutations, chemotherapy, and radiation therapy;        b) hyperthermia; and        c) deregulation of microtubules caused by some chemotherapeutic drugs, e.g., treatment using taxol or Vinca alkaloids.        
When activated, p53 causes cell growth arrest or a programmed, suicidal cell death, which in turn acts as an important control mechanism for genomic stability. In particular, p53 controls genomic stability by eliminating genetically damaged cells from the cell population, and one of its major functions is to prevent tumor formation.
p53 is inactivated in a majority of human cancers (A. J. Levine et al., Br. J. Cancer, 69, p. 409 (1994) and A. M. Thompson et al., Br. J. Surg., 85, p. 1460 (1998)). When p53 is inactivated, abnormal tumor cells are not eliminated from the cell population, and are able to proliferate. For example, it has been observed that p53-deficient mice almost universally contract cancer because such mice lack a gene capable of maintaining genomic stability (L. A. Donehower et al., Nature, 356, p. 215 (1990) and T. Jacks et al., Curr. Biol, 4, p. 1 (1994)). A loss or inactivation of p53, therefore, is associated with a high rate of tumor progression and a resistance to cancer therapy.
p53 also imparts a high sensitivity to several types of normal tissue subjected to genotoxic stress. Specifically, radiation therapy and chemotherapy exhibit severe side effects, such as severe damage to the lymphoid and hematopoietic system and intestinal epithelia, which limit the effectiveness of these therapies. Other side effects, like hair loss, also are p53 mediated and further detract from cancer therapies. These side effects are caused by p53-mediated apoptosis, which maps tissues suffering from side effects of cancer therapies. Therefore, to eliminate or reduce adverse side effects associated with cancer treatment, it would be beneficial to inhibit p53 activity in normal tissue during treatment of p53-deficient tumors, and thereby protect normal tissue.
However, loss of p53 activity in tumors is associated with faster tumor progression and resistance to cancer treatment. Therefore, conventional theories dictate that suppression of p53 would lead to disease progression and protection of the tumor from a cancer therapy. Consequently, prior investigators attempted to restore or imitate the function of p53 in the prevention or treatment of a cancer.
Inactivation of p53 has been considered an undesirable and unwanted event, and considerable effort has been expended to facilitate cancer treatment by restoring p53 function. However, p53 restoration or imitation causes the above-described problems with respect to damaging normal tissue cells during chemotherapy or radiation therapy. These normal cells are subjected to stress during cancer therapy, which leads the p53 in the cell to cause a programmed death. The cancer treatment then kills both the tumor cells and the normal cells. A discussion with respect to suppression of p53 in various therapies is set forth in the publication, E. A. Komarova and A. V. Gudkov, “Could p53 be a target for therapeutic suppression?,” Seminars in Cancer Biology, Vol. 8(5), pages 389-400 (1998), incorporated herein by reference.
In summary, p53 has a dual role in cancer therapy. On one hand, p53 acts as a tumor suppressor by mediating apoptosis and growth arrest in response to a variety of stresses and controlling cellular senescence. On the other hand, p53 is responsible for severe damage to normal tissues during cancer therapies. As disclosed herein, the damage caused by p53 to normal tissue made p53 a potential target for therapeutic suppression. In addition, because more than 50% of human tumors lack functional p53, suppression of p53 would not affect the efficacy of a treatment for such tumors, and would protect normal p53-containing tissues.
The adverse effects of p53 activity on an organism are not limited to cancer therapies. p53 is activated as a consequence of a variety of stresses associated with injuries (e.g., burns) naturally occurring diseases (e.g., hyperthermia associated with fever, and conditions of local hypoxia associated with a blocked blood supply, stroke, and ischemia) and cell aging (e.g., senescence of fibroblasts), as well as a cancer therapy. Temporary p53 inhibition, therefore, also can be therapeutically effective in: (a) reducing or eliminating p53-dependent neuronal death in the central nervous system, i.e., brain and spinal cord injury, (b) the preservation of tissues and organs prior to transplanting, (c) preparation of a host for a bone marrow transplant, and (d) reducing or eliminating neuronal damage during seizures, for example.
Activated p53 induces growth arrest, which often is irreversible, or apoptosis, thus mediating damage of normal tissues in response to the applied stress. Such damage could be reduced if p53 activity is temporarily suppressed shortly before, during, or shortly after, a p53-activating event. These and other p53-dependent diseases and conditions, therefore, provide an additional area for the therapeutic administration of temporary p53 inhibitors.
p53 also plays a role in cell aging, and, accordingly, aging of an organism. In particular, morphological and physiological alterations of normal tissues associated with aging may be related to p53 activity. Senescent cells that accumulate in tissues over time are known to maintain very high levels of p53-dependent transcription. p53-dependent secretion of growth inhibitors by senescent cells accumulate in aging tissue. This accumulation can affect proliferating cells and lead to a gradual decrease in overall proliferative capacity of tissues associated with age. Suppression of p53 activity, therefore, is envisioned as a method of suppressing tissue aging.
However, there are several important objectives that should be satisfied before a therapy involving suppression of p53 is implemented, for example:                (i) providing a p53 inhibitor that is sufficiently efficacious in vivo for practical administration as a therapeutic drug (i.e., inhibits p53 activity in a micromolar (μm) range of concentrations);        (ii) providing a p53 inhibitor that has a sufficiently low toxicity for use in therapy, and also does not cause undesirable side effects at concentrations sufficient to inhibit p53 activity;        (iii) exhibiting a p53 inhibition that is reversible because long-term p53 inactivation can significantly increase the risk of cancer;        (iv) during temporary p53 inhibition, the cells should recover from the applied stress and the p53-activating signal should be eliminated or reduced, otherwise restoration of p53 activity while the p53-activating signal is active could result in cell damage; and        (v) the p53 suppression therapy is not associated with a dramatic increase in the frequency of cancer development, i.e., the therapeutic inhibitors target p53-mediated control of cellular response to stress, but do not affect p53-mediated control of oncogene transformation.        
Until the present invention, p53 inhibitors useful in therapeutic applications have not been disclosed. A potential therapeutic inhibitor of p53 is a compound that acts at any stage of the p53 signaling pathway, and leads to functional inactivation of a p53-mediated response (i.e., blocking of p53-dependent growth arrest, apoptosis, or both). Prior investigators did not consider therapeutic p53 inhibitors because therapeutic p53 suppression was considered a disadvantage leading to the onset and proliferation of cancerous tumors. The present invention, therefore, is directed to the therapeutic and temporary inhibition of p53 activity, and to compounds capable of such inhibition.